The present invention relates to molds, particularly for the molding of tires, and in particular to venting techniques used in these molds in order to permit the air imprisoned between the mold and a raw tire blank to escape to the outside.
Molds for tires are frequently equipped with a number of vents arranged at suitable places. These vents are, for instance, generally holes of small diameter connecting the inner cavity of the mold with the outside of the mold. As it is not possible to produce holes of extremely small diameter due to inherent limitations in the machining techniques, it results that the diameter of these holes is generally sufficient to permit rubber to flow, at least slightly, into said holes when the tire is molded. This is the reason why small burrs of rubber protruding slightly from the outer surface can be noted on the surface of numerous tires.
It has been attempted to reduce the dimensions of the vents to a size which is sufficiently small to prevent the raw rubber from flowing into the vent spaces while permitting the air to escape to the outside of the mold. In order to reduce the diameter of the vents beyond what it is possible to do by techniques of direct drilling on the mold, it has been proposed to pierce a hole of relatively large diameter in the mold and to insert into said hole a vent which, in its turn, has a hole or slit or space of extremely reduced size. By way of example, reference may be had to U.S. Pat. No. 4,447,197 which describes a vent of this type. Another example is described in U.S. Pat. No. 3,854,852, in which there can be noted a vent of "valve" type which closes at the time that the raw rubber comes into contact with the head of the valve, after having allowed the air imprisoned between the raw rubber and the mold to escape.
The vents known from the prior art have numerous drawbacks. While some of them permit the air to escape while effectively preventing the appearance of molding burrs on the vulcanized tire, their satisfactory operating life remains, in general, very short. This makes it necessary to repair the mold far too frequently. It is therefore necessary to remove it regularly from the vulcanization press and shut the press down for the time necessary for the repair operations. Furthermore, it is not always possible to restore the venting capacity of these vents, and they must therefore be replaced rather regularly. This leads to an increase in the industrial cost price of these venting techniques. Furthermore, in the case of vents of the valve type, the initial cost price of such vents is relatively high.
With regard to the tread of tires, recourse is had to different means of venting, namely the implanting of vents such as described above, the production of simple drillings, or else one also makes use of the slits which are present between the different parts of the mold (planes of joints between parts) in order to conduct the air from the inside of the molding cavity to the outside of the mold. If necessary, the places where the planes of joints between the different parts of the mold are judiciously selected with respect to the tread pattern of the tire so that, whenever possible, a slit is present at the place where it is necessary to vent the mold.
By way of illustration of this last-mentioned technique, reference may be had to U.S. Pat. No. 5,234,326. In that case, for certain tread patterns, it is possible to develop a complete venting of the tire mold which does not require the drilling of vents. In the case of other tire patterns, however, it is not possible to achieve this purpose. Therefore, even in the tread portion of a tire mold, it is not always possible to avoid having recourse to drillings or vents.
A molding of better and better quality is always desired. The molding burrs which appear on the surface of the tire, whether in the form of small strips or of small bits, are harmful even in the event that the amount of rubber which has flowed out remains very small, since the appearance of the tire is such that the quality perceived by the buyer is considered poor.
Therefore, the problem is always present of succeeding in organizing a complete venting of a tire mold and of succeeding in doing so by dependable means, that is to say means which require only very little maintenance in industrial operation. The problem also arises of designing a less expensive venting, the maintenance of which is as simple as possible, in order that it can be adopted very easily by tire manufacturing workshops without the need of sending the mold back to a mold-construction workshop for it to undergo renovation there.